The present invention relates to an exposure technique using a charged particle beam such as an electron beam or, in particular, to an exposure apparatus and an exposure method useful for detecting partial unevenness or partial distortion on the surface of a specimen (specifically, a wafer) to be exposed.
With the increase in density of integrated circuits in recent years, a new exposure method using a charged particle beam such as an electron beam or an ion beam or an exposure method using X-rays has been studied and realized as a method to replace photolithography which has long been the mainstay for forming fine patterns. Among these new methods, the electron beam exposure for forming a pattern using an electron beam has a great feature in that the sectional area of the electron beam can be reduced to the order of several tens of nm and can form a pattern as fine as not more than 1 xcexcm.
The electron beam exposure apparatus is for exposing a finer pattern than photolithography, and for an exposure to be effected with high accuracy, a beam having a predetermined sectional shape is required to be focused to assure accurate irradiation at the desired position on a wafer. A change of the surface position (i.e. the height) of the wafer causes an out-of-focus condition and the displacement of the exposure position. The out-of-focus condition and the displacement deteriorate the accuracy of the exposure pattern.
In photolithography, it is common practice to expose one chip (die) in one shot. The exposure is effected on the assumption that the wafer height is the same within the exposure range of one chip. In other words, a change in height, if any, within the exposure range of one chip cannot be adjusted. For this reason, the change in height within the exposure range of one chip has not been measured. In the electron beam exposure apparatus, in contrast, the range exposed in one shot is smallest for the single beam method, and increases for the variable rectangle method, the block exposure method and the BAA exposure method, in that order. Nevertheless, the maximum exposure range is limited to a square of several tens of xcexcm. If the wafer height is measured and adjusted for each exposure range, the exposure of higher accuracy is made possible. For this adjustment to be carried out, it is necessary to accurately measure the wafer position, i.e. the height of the wafer surface where the beam is to be irradiated.
A well known conventional method of measuring the wafer surface height includes a method using an electron beam or a light beam. In the method using an electron beam, a reference pattern formed on the wafer for exposure alignment is scanned while changing the focal point of the electron beam and the beam reflected in the process is detected. It is determined that the beam is exactly focused at the time point when the reflected beam detection signal has undergone the sharpest change. This method poses the problem, however, that the measurement is possible only for a portion having a reference pattern and that a multiplicity of scanning operations are actually required by changing the focal point, thus requiring a long time for measurement.
Another category of methods known for measuring the wafer surface height uses an optical height measuring apparatus. This category includes a method for detecting the displacement of the focal point by use of astigmatism or a knife edge and detecting the height of the wafer surface from the position controlled by feedback in such a manner as to secure a focal point on the wafer surface, or a method in which a light beam is radiated on the wafer surface and the displacement of the reflected light beam is detected thereby to detect the height or height change of the wafer surface. In any case, the height is measurable only at a point on the wafer. For measuring the height distribution over the entire wafer surface, the wafer is moved with a stage, the height is measured at a plurality of points on the wafer, and the measurement points are interpolated by the spline function or the Wentzel function thereby to calculate a curved surface indicating the height distribution over the entire surface. The height may be measured continuously while moving the wafer, in which case the height is measured along the locus of movement so that the curved surface indicating the height distribution over the entire wafer surface is also calculated by interpolation. According to this curved surface, the focus and the deflection efficiency of the electron beam for exposure are set to adjust the height of the wafer surface.
The conventional methods of height measurement described above pose no problem in the case where an unevenness or distortion occurs over a comparatively wide area on the wafer surface, i.e. as far as the unevenness is such that it can be approximated by a smooth curved surface. In such cases, the wafer surface height can be adjusted with high accuracy.
In the case where partial unevenness or partial distortion is present in a comparatively small range of the wafer surface, however, the problem is that the points where the partial unevenness is present cannot always be identified.
The partial unevenness is often caused by foreign matter such as dust or wafer parts caught between the electrostatic chuck (wafer chuck) and the wafer when the latter is fixed on the stage. Such foreign matter is either stuck on the stage and changes the partial height at the same position repeatedly or is delivered with the wafer after the exposure process and is thus removed before the next wafer is fixed on the stage. In any way, the problem is encountered that the presence of partial unevenness on the wafer surface cannot be ascertained. In the case where partial unevenness or the like is present on a given wafer, it is of prime importance that the fact can be ascertained and that some preventive measure can be taken against the particular wafer.
Other means conceivable for detecting partial unevenness or the like of the wafer surface is to increase the number of measurement points by finely moving the stage carrying the wafer in the conventional height measurement process. This method, however, cannot accurately identify whether the detected partial unevenness is caused by the wafer itself or due to the movement of the stage and the resulting height change. The problem, therefore, is that the determination of the detection result remains ambiguous. Also, the great number of measurement points consumes a considerable length of time and reduces the throughput of the apparatus as a whole.
The present invention has been created in view of the problems of the prior art described above, and the object thereof is to provide a charged particle beam exposure apparatus and an exposure method in which a partial unevenness developed on the surface of a specimen to be exposed can be positively detected and thus the height of the specimen surface to be exposed can be accurately adjusted, thereby contributing to a highly accurate exposure and an improved yield.
A charged particle beam exposure apparatus and method according to the present invention use a height measuring unit for measuring the height distribution in a predetermined range on a specimen to be exposed at not less than a predetermined density with the specimen loaded in the apparatus.
Specifically, according to one aspect of the present invention, there is provided a charged particle beam exposure apparatus comprising a charged particle beam source for generating a charged particle beam, a shaper for shaping the charged particle beam, a deflector for changing the position at which the charged particle beam is radiated on the specimen to be exposed, a projector for projecting the charged particle beam on the specimen to be exposed, a control unit for controlling the deflector and the projector at the time of exposure, means for plotting a pattern on the specimen to be exposed by the charged particle beam projected and deflected appropriately, a stage for moving the specimen to be exposed within the apparatus, and a height measuring unit for measuring the height distribution in a predetermined range of the specimen to be exposed at not less than a predetermined density while the specimen is loaded in the apparatus.
The apparatus preferably further comprises a detected height processing unit for changing the measuring points of the specimen to be exposed for the height measuring unit by moving the specimen with a stage, combining the measurements at the various points and calculating the height distribution over the entire surface of the specimen to be exposed.
According to another aspect of the invention, there is provided a charged particle beam exposure method for plotting a pattern on a specimen to be exposed by a charged particle beam converged and deflected appropriately on the specimen, comprising the steps of, before plotting the pattern by the charged particle beam, loading the specimen on a stage of an exposure apparatus, measuring the height distribution in a predetermined range of the specimen at not less than a predetermined density, calculating the height distribution over the entire surface of the specimen by changing the measuring points on the specimen moved with the stage and combining the measurements at the various points, determining the allowability of the partial height change of the specimen based on the measurements, and exposing the specimen in the case where the determination of the allowability is satisfactory.
In the configuration for the charged particle beam exposure apparatus and method according to this invention, the height distribution in a predetermined range of the specimen surface is measured by a height measuring unit with the specimen loaded in the apparatus when a pattern is not plotted (the exposure process is not executed) by the charged particle beam. A partial unevenness on the specimen to be exposed which has thus far been difficult to identify with the conventional height measuring method can be positively detected. As a result, the height measurement and the height adjustment (by focusing or setting the deflection efficiency of the charged particle beam at the time of exposure) based on the height measurement of the specimen to be exposed can be carried out with high accuracy, thereby making it possible to attain a highly accurate exposure and improve the yield.
The apparatus preferably further comprises an alarm unit for issuing an alarm notifying a height change, if any is detected, exceeding a predetermined allowance within a predetermined range of the exposure area on the specimen to be exposed and a storage unit for storing information on the position of the stage corresponding to the position on the specimen where the height has changed. An alarm is issued, for example, in accordance with the result of determination as to whether an estimation value K defined as K=xcex94H/xcex94L has exceeded a predetermined value, where xcex94L is the distance between two points in the exposure area on the specimen to be exposed and xcex94H is the difference of height between the two points. Further, the apparatus preferably comprises a determination unit for determining whether the height change exceeding a predetermined allowance is caused by the specimen itself or the stage, from the information on the stage position where the alarm is issued on a plurality of specimens.
The height measuring unit includes, for example, a light beam emitter for emitting a plurality of light beams parallel to each other in a two-dimensional array and a two-dimensional light detector for detecting a plurality of light beams reflected from the surface of the specimens to be exposed. The light beam emitter is configured either in such a manner as to include a plurality of light beam sources in a two-dimensional array or in such a manner as to include a laser beam source, a lens for converting the laser beam emitted from the laser beam source into a parallel beam and an aperture array having a plurality of apertures in a two-dimensional array for splitting the parallel beam into a plurality of beams corresponding to the apertures.
The height measuring unit is an interference unit for measuring the height distribution from the interference fringe generated by interference between the light beam reflected from a reference plane and the light beam reflected from the surface of the specimen to be exposed. This interference unit may be a Twyman-Green interferometer including a laser beam source, a lens for converting the laser beam emitted from the laser beam source into a parallel beam, a reference reflection surface, a beam splitter for splitting the parallel beam into a beam incident to the surface of the specimen to be exposed and a beam incident to the reference reflection surface and combining the beam reflected from the specimen surface and the beam reflected from the reference reflection surface, and a two-dimensional light detector for detecting the interference pattern obtained in the form of a combined light beam.
The above and other objects, features and advantages will be described in detail below with reference to embodiments.